Because it's so old (IOS version 12.0.(5.2)-XU) and primitive, I discovered that.. first, the port mirroring setup page on Wikipedia doesn't give a working configuration. For old switches like these, it's a multi-step process, e.g.
#port monitor fa0/2
#port monitor fa0/3
#show port monitor
to set up (and verify) that FastEthernet 0/1 is used to monitor interfaces 0/2 and 0/3. Far more than the two-line configuration in the Wikipedia page!!
Another important caveat is that Catalyst 2900 and 3500 switch ports are not auto-sensing MDI/MDI-X. To connect one of these switches to another switch, you actually need a cross-cable! how 1990's!!
I still haven't verified my SPAN port is working. Will do that tomorrow.